U.S. Pat. No. 2,993,025, granted July 18, 1961 to Alsup et al; U.S. Pat. No. 3,027,352, granted Mar. 27, 1962 to Walling et al; U.S. Pat. No. 3,743,614, granted July 3, 1973 to Wolters et al; U.S. Pat. No. 3,787,353, granted Jan. 22, 1974 to Ishii et al; U.S. Pat. No. 3,960,984, granted June 1, 1976 to Kohan; and U.S. Pat. No. 4,098,843, granted July 4, 1978 to Johnson, all disclose various polyoxymethylene compositions, both those based on homopolymers and those based on copolymers, and various techniques for stabilizing such compositions. Alsup discloses stabilization of polyoxymethylene compositions by blending therewith a synthetic polyamide. Walling discloses that the thermal stability of certain polyoxymethylene copolymer compositions is improved as compared with corresponding homopolymer. Wolters discloses stabilization of polyoxymethylene compositions by blending therewith a combination of an alkaline earth metal compound and an ester of an (alkyl-hydroxyphenyl)-carboxylic acid with a polyol. Ishii discloses stabilization of polyoxymethylene compositions by blending therewith a compound of the formula R(NHCOCH.sub.2 X).sub.n where R is a hydrocarbon group, X is a cyano or carbamoyl group, and n is 2-6. Kohan discloses stabilization of polyoxymethylene compositions by blending therewith an amide oligomer. Johnson discloses stabilization of polyoxymethylene compositions by blending therewith a dispersion of polyamide in a carrier resin.
The polyoxymethylene compositions described in the patents cited above are examples of compositions that can be modified according to the present invention to give compositions characterized by extraordinary toughness.
Various additives have been used with polyoxymethylene compositions to improve the toughness or impact strength of such compositions. None have been able to achieve the extraordinary degree of toughness reached in the present invention. In addition, the compositions of the present invention achieve the extraordinary degree of toughness with a minimal sacrifice of other desirable properties of such compositions.
U.S. Pat. No. 3,795,715, granted Mar. 5, 1974 to Cherdon et al, discloses improving the impact strength of polyoxymethylene compositions by blending therewith 0.1-10 parts by weight of a polymer having (a) an average molecular weight of 1,000-1,000,000, (b) a softening temperature below the crystalline melting point of the polyoxymethylene, and (c) a second order transition temperature of -120.degree. to +30.degree. C., this latter polymer being present in the form of particles of 0.1-5 microns in diameter. Such copolymers include polyethylene, ethylene/propylene copolymers, (meth)acrylic acid ester homo- or copolymers, diene homo- or copolymers, and vinyl ester homo- or copolymers. Modest improvements in impact strength as measured by a falling weight test are disclosed.
U.S. Pat. No. 4,277,577, granted July 7, 1981 to Burg et al, discloses polyoxymethylene compositions similar to those disclosed by Cherdon, above, except that they also contain 0.01-20 weight percent of a third polymeric component which can be a segmented thermoplastic copolyester or a polyurethane.
U.S. Pat. No. 3,850,873, granted Nov. 26, 1974 to Wurmb et al, discloses improving the physical properties (including impact strength) of glass fiber reinforced polyoxymethylene compositions by blending therewith 0.5-10 weight percent of a high molecular weight thermoplastic polyurethane. The polyurethane is not defined, except by two examples. Modest improvements in inpact strength are disclosed.
British Pat. No. 1,381,106, published Jan. 22, 1975, discloses improving the impact strength of polyoxymethylene copolymers by copolymerizing an elastomeric terpolymer having a molecular weight of at least 100,000 therewith via urethane, ureide, thiourethane or thioureide linkages.
U.S. Pat. No. 3,476,832, granted Nov. 4, 1969 to Pritchard, discloses improving the impact resistance of thermoplastic oxymethylene polymers by blending therewith up to 20% of a rubbery polymeric material having a glass transition temperature below 0.degree. C. Preferably, the rubbery material is dispersed as particles having an average diameter of less than 20 microns. Thermoplastic polyurethanes are not mentioned, and only modest increases in impact strength are reported.
U.S. Pat. No. 3,642,940, granted Feb. 15, 1972 to Burg et al, discloses improving the impact strength of polyoxymethylene molding compositions by blending therewith a two phase mixture of an elastomeric and a hard polymer. Thermoplastic polyurethanes are not mentioned as possible elastomeric components of the two phase mixture.
U.S. Pat. No. 3,749,755, granted July 31, 1973 to Bronstert et al, discloses improving the impact resistance of thermoplastic polyoxymethylene molding compositions by blending therewith an elastomeric graft copolymer having a glass transition temperature below -20.degree. C. Thermoplastic polyurethanes are not mentioned.
Other examples of various additives, other than thermoplastic polyurethanes, for improving the impact resistance of polyoxymethylene compositions are disclosed in U.S. Pat. No. 3,975,459, granted Aug. 17, 1976 and U.S. Pat. No. 4,017,558, granted Apr. 12, 1977, both to Schmidt et al; Japanese Patent Publication 48-15,954, published Feb. 28, 1973; and Japanese Patent Publication 52-019,752, published Feb. 15, 1977.
None of the reference discussed or listed above report the extraordinary toughness which can be achieved by the techniques of the present invention.